Marine jet drive systems for watercraft generally comprise a water conduit disposed toward the rear of the watercraft which conduit comprises an intake portion, a pump portion and a discharge portion. A pump impeller within the pump portion is generally driven by a shaft which extends from a prime mover through the intake portion to the pump portion. Water from under the watercraft enters the intake portion and passes to the pump portion. The water pump impeller in the pump portion increases the energy of the incoming water which then flows from the pump into a discharge chamber in the discharge portion. Water exits from the discharge portion as an organized jet providing propulsion for the watercraft.
In conventional jet drive systems, atmospheric pressure is the only force pushing water into the intake portion when the craft is at rest and the eye, or front face of the pump impeller, is above the water level or only partially submerged, thus the net inlet head at the pump inlet is less than atmospheric. Accordingly, at start-up, the pump impeller must first generate a sufficient suction head to draw water from the intake into the pump. This results in a delay or slippage until full water flow through the pump is achieved. In addition, because of initial uneven water flow through the pump, cavitation, which is caused by the formation and collapse of partial vacuums in the flowing water, occurs around the impeller with the consequences of a reduction in water flow thrust as well as damage to the impeller surface.
Prior systems have sought to alleviate these problems by providing marine jet pumps with multiple stages in which a first stage impeller is driven at a slower speed or in counter-rotation to a second and/or subsequent stage impellers. In general, the first stage impeller is larger than the subsequent stage impellers so as to build up water flow through the pump in successive stages of low pressure rise. In addition, such systems house all of the impellers within the pump portion in a manner whereby the first stage impellers are enclosed at their outer perimeter such that water flow is restricted to axial flow through the impeller.
Examples of such prior systems include U.S. Pat. No. 3,328,961, which employs a pump assembly having a larger, slower rotating first stage impeller and a smaller, faster rotating second stage impeller. The impellers are mounted on and driven by separately rotating concentric shafts and are peripherally closely adjacent to the internal surface of the pump housing.
U.S. Pat. No. 3,405,526 describes a multiple stage hydraulic jet propulsion apparatus which combines a slower rotating first stage axial flow impeller and a faster rotating second stage mixed flow impeller. As with the '961 patent, the impellers are mounted on separately rotating concentric shafts within the pump housing so that the peripheral edges of the impeller vanes are closely adjacent to the walls of the housing.
In U.S. Pat. No. 3,531,214 a multistage jet pump is disclosed in which a plurality of successively smaller impellers are radially driven from a common shaft by means of gears on the shaft which mesh with gear teeth on peripheral rings of the impellers. The effect is that the impellers are fully enclosed peripherally by their respective ring gears and, due to the differing gear ratios, are driven at successively higher speeds.
U.S. Pat. No. 5,634,831 discloses a water jet propulsion unit which employs two counter-rotating impellers within the pump. Mounted on concentric, counter-rotating shafts, the impellers are calibrated so that any radial flow created by the upstream impeller is converted into axial flow by the downstream impeller. However, like the other prior art, the impellers of this unit are within the pump housing with their peripheral edges closely adjacent to the housing wall.
Thus, the prior art seeks to solve the problems of delay and cavitation in marine jet drive systems by complex mechanisms. Although these prior art systems may be somewhat effective at improving the efficiency and performance of jet drives once the watercraft powered by them are in motion, the first stage impellers of these systems remain at a position where the eye of the impeller is above the water level or, at best, only partially submerged and the net inlet head is less than atmospheric. Thus, the first stage impeller of the prior art multistage pumps suffers from the same problems that its presence seeks to cure in the subsequent stages.